1. Field of the Invention
The present invention relates to a method of controlling the air/fuel ratio for an internal combustion engine mounted on a vehicle.
2. Description of Background Information
Feedback type air/fuel ratio control systems for an internal combustion engine are known from, for example, Japanese Patent Publication No. 55-3533, which systems perform a feedback control of the air-fuel ratio of the mixture to be supplied to the engine wherein the concentration of an exhaust gas component in the exhaust gas such as the oxygen concentration is detected by an exhaust gas component concentration sensor (for example, an oxygen concentration sensor), and the volume of air or fuel to be supplied to the engine is regulated in response to the detected value obtained by the exhaust gas component concentration sensor, in order to attain the purification of the exhaust gases, the improvement of the fuel economy, etc.
In this type of conventional air/fuel ratio control systems, a PI (Proportional and Integral) control is normally adopted in which a base value of the air/fuel ratio control indicating the amount of the intake side secondary air is determined in accordance with a plurality of operation parameters relating to the engine load, whether the air/fuel ratio of the supplied mixture is leaner or richer than a desired air/fuel ratio such as the stoichiometric air/fuel ratio is determined from the output signal level of the exhaust gas component concentration sensor, a correction value of the air/fuel ratio is increased or decreased by a proportional value or an integral value at predetermined intervals in accordance with the result of the determination, and the base value of the air/fuel ratio is corrected in accordance with the correction value of the air/fuel ratio.
On the other hand, due to a change caused by the lapse of time, or deterioration, of the carburetor, it is general that the base air/fuel ratio value of the carburetor becomes to deviate from a predetermined value so that the base value of the air/fuel ratio control will not correspond to the desired air/fuel ratio, thereby causing an error.
Therefore, there are some methods wherein a learning control is executed in which a base correction value for correcting the error of the base value of air/fuel ratio control is calculated for every operational region during the feedback control of air/fuel ratio, and the calculated base correction value is stored in a memory device such as a RAM and is renewed, to accomplish an improvement of the accuracy of the air/fuel ratio control operation.
The base correction value or an amount of change in the base correction value is normally calculated every time the air/fuel ratio of the supplied mixture turns over with respect to the desired air/fuel ratio, and the renewal of the base correction value is performed by storing the newest base correction value when the operational region changes.
On the other hand, when te engine is idling, the speed of combustion is slower than other operational states, and the speed of change in the air/fuel ratio correction value remains small because the proportional and integral control amounts are small. Therefore, the period from one turn-over of the air/fuel ratio of the supplied mixture with respect to the desired air/fuel ratio to the next turn-over becomes long, and the calculation of the base correction value is not performed frequently.
Therefore, when the engine speed is low, such as in the idling state, there has been a drawback that the renewal of the base correction value will be delayed. Especially , when the number of renewal is not sufficiently large, such as with a vehicle having a small total mileage, or in the event that characteristic of the carburetor has changed suddenly, the delay of the renewal of the base correction value will adversely affect on the feedback control of the air/fuel ratio so that the air/fuel ratio will be biased on the rich or lean side, to cause degradation of the performance of the exhaust gas purifying operation.
On the other hand, with respect to internal combustion engines mounted on a vehicle, there is a condition that the engine is once stopped after running long time at high speed for example, and subsequently the engine is started again under the condition of high temperature. Such a condition can produce the percolation phenomenon by which a large quantity of fuel is pushed out from the nozzle of the carburetor by the pumping function of bubbles of vaporized fuel which has been generated from the fuel in the float chamber of the carburetor and the fuel supply passage due to the heat during the stop of the engine. Therefore, the air/fuel ratio of the supplied mixture is enriched immediately after the hot starting of the engine, and there has been a problem that the emission of unburnt components such as CO (carbon monoxide) and HC (hydrocarbons) is increased.
Furthermore, if the temperature of the intake air is high during an idling state after the hot starting of the engine, the air/fuel ratio becomes excessively rich because of the reduction in the density of air in addition to the vapor of fuel from the carburetor, and moreover, due to the reduction in the replenishment efficiency in the combustion chamber, the air/fuel ratio becomes over-rich. Since the correction amount of air/fuel ratio by the proportional-integral control operation remains small and the speed of correction of the over-rich by the supply of the intake side secondary air is also small during the idling state, such an idling state after the hot starting of the engine as mentioned above can cause a drop of the engine rotational speed. The over-rich air/fuel ratio is continued until the vapor of fuel once adhered in the intake pipe is completely absorbed, therefore, the emission of CO and HC especially, is increased during such a period.